It is known that when titanium dioxide is exposed to light, a photocatalytic action takes place and it is possible to sterilize bacteria, etc., and to decompose organic compounds such as odor components, for example.
Without ultraviolet rays, photocatalysts cannot perform sufficient catalytic action. In outdoor environments, sunlight is available. Even in shaded areas, there is a sufficient amount of ultraviolet rays available for catalytic reaction, and a reasonable level of effect can be expected. In indoor environments on the contrary, aside from some sunlight available from windows, light from lighting fixtures such as fluorescent lamps is the only source available, and the light from fluorescent lamps is not enough to obtain sufficient photocatalytic action.
Photocatalysts such as titanium dioxide are marketed in the form of powder. The powder is then mixed with and supported by resin or paper material, and becomes available as filters for example.
As an example, JP-A 2004-44882 (Prior Literature 1) discloses an air purifying apparatus which makes use of the above-described photocatalyst: The photocatalyst is supported on an inner surface of a reactor vessel through which air is passed. The air purifying apparatus is tubular, and includes a fan which moves air from one opening to the other of the tube, and a light source which throw light to the inner surface where the photocatalyst is supported.
Further, JP-A 2000-119995 Gazette (Prior Literature 2) discloses antibacterial paper which includes titanium oxide powder dispersed and bonded uniformly on a surface layer of paper material. In a paper making vat, the titanium is dispersed in the paper material, and then the paper material and the powdery titanium dioxide particles are gathered evenly on a surface of a paper making frame.
According to the air purifying apparatus disclosed in Prior Literature 1, air in the tubular reactor vessel moves only in an axial direction of the tubular vessel. Therefore, only a part of the air which moves through the tubular reactor vessel will pass a proximity to the inner surface where the photocatalyst is supported. In addition, since the photocatalyst is supported only on the inner surface of the tube, the amount of the photocatalyst to act on the air is limited, and therefore it is not possible to expect sufficient air purification effect.
Another problem can be the fan which moves the air. With a small area of surface which supports the photocatalyst, air moving at a high velocity can stay near the photocatalyst only for a small amount of time, making it difficult to provide sufficient air purification effect. Using a fan may also pose noise problems.
According to the antibacterial paper disclosed in Prior Literature 2, it is possible to support titanium dioxide particles reliably, without using a binder. However, because the titanium dioxide particles must be put in a paper making vat so that titanium dioxide is scooped together with the fiber material and supported in the formed sheet during the paper making process, the method requires a large amount of titanium dioxide.
On the other hand, photocatalytic action can be expected only in a range where light can reach, and light is most available on the surface. However, according to the invention disclosed in Prior Literature 2, titanium dioxide is supported uniformly in the direction of depth. Photocatalytic action can only be expected from titanium dioxide which is present in a surface region, and most of the supported titanium particles cannot perform the photocatalytic reactions. This means poor efficiency with respect to the amount of supported photocatalyst.